Method for examining a solidified and/or hardening material using ultrasound, receptacle and ultrasound sensor for carrying out the method

ABSTRACT

A method is described for examining a solidifying and/or hardening material such as cement, concrete or the like, using ultrasound waves emitted by an ultrasound transmitter, which penetrate the solidifying and/or hardening material, are continuously measured and analyzed. During solidification and/or hardening of the material, the signal shapes of the ultrasound waves penetrating the material, are recorded. The change with time of the compression wave velocity and/or the relative energy of the ultrasound waves and/or the frequency spectra of the ultrasound waves is extracted from the ultrasound wave shapes during the entire course of solidification and/or hardening of the material. This change with time of the compression wave velocity and/or the relative energy of the ultrasound waves and/or the frequency spectra of the ultrasound waves is approximated through a compensating function, preferably the Boltzmann function. The free parameters of the compensation function are associated with material properties and permit comparison of a current measurement with reference values of these parameters to permit determination of material properties of the examined material. A receptacle and an ultrasound transmitter for carrying out the method also are described.

BACKGROUND OF THE INVENTION

The invention concerns the examination of a solidifying and/or hardeningmaterial, such as cement, concrete or the like, using ultrasound waves,emitted from an ultrasound transmitter to an ultrasound receiver, whichpenetrate the material and are continuously measured and analyzed.

PRIOR ART

Such examinations are known e.g. from the publication “KontinuierlicheUltraschallmessungen während des Erstarrens and Aushärtens von Beton”(continuous ultrasound measurements during solidification and hardeningof concrete) by Chr. U. Grosse and H.-W-Reinhardt in Otto-Graf-Journal,Vol. 5, 1994.

Ultrasound waves can penetrate a material without causing damage therebybeing influenced by the elastic properties of the material, whichproduces information about the elastic properties.

With concrete, these are e.g. its current solidification and hardeningstate, composition (grading curve, water-cement value etc.) and theentrained air content and possibly utilized additional means.

In industrial construction e.g., determination of solidification startand end of cement paste according to DIN EN 196, part 3, is carried outthrough the Vicat method. A measurement of this type is not possiblewith concrete due to the aggregate and is therefore not provided in theabove-mentioned standard. Examination methods for unset concrete havebeen, on the one hand, consistency measuring methods such as thepropagation test and compacting test according to DIN 1048 part 1, thepenetrometer according to ASTM C-403 and the setting test according toDIN ISO 4109. On the other hand, there is the air content measurementaccording to DIN 1048 part 1 including pressure compensation method andfurthermore methods for determining the water content.

The latter methods permit only individual measurements at fixed pointsin time and give information about a certain property. It is notpossible to obtain detailed information about the composition of thematerial nor about the further hardening of the material aftersolidification.

OBJECT OF THE INVENTION

It is therefore the underlying purpose of the invention to providereliable use of an ultrasound test method in industrial practice andpermit easy continuous monitoring of the state of a solidifying and/orhardening material.

SUBJECT MATTER AND ADVANTAGES OF THE INVENTION

This object is achieved by a method for examining a solidifying and/orhardening material such as cement, concrete or the like, usingultrasound waves emitted by an ultrasound transmitter, which penetratethe solidifying and/or hardening material, are continuously measured andanalyzed, comprising the following method steps:

i) during solidification and/or hardening of the material, the signalshapes of the ultrasound waves penetrating the material are recorded;

ii) The change with time of the compression wave velocity and/or therelative energy of the ultrasound waves and/or the frequency spectra ofthe ultrasound waves is extracted from the ultrasound wave shapes duringthe entire course of solidification and/or hardening of the material.

iii) This change with time of the compression wave velocity and/or therelative energy o the ultrasound waves and/or the frequency spectra ofthe ultrasound waves is approximated through a compensating function,preferably the Boltzmann function.

iv) the free parameters of the compensation function are associated withmaterial properties.

v) the free parameters of the compensation function permit comparison ofa current measurement with reference values of these parameters topermit determination of material properties of the examined material.

Automatic measuring and analysis of the data is largely possible andinformation about the material itself can be obtained already during thesolidifying/hardening phase.

For the measurement, the material to be examined is introduced into areceptacle and compacted. The opposing sides of the receptacle areprovided with a preferably broad-band (i.e. adequately linear frequencyresponse function over a broad spectral range) ultra sound transmitterand a corresponding receiver. Same transforms the acceleration signalinto a voltage signal and transmits it to a computer-controlledanalog-digital transformer card which stores the signal in digital formthereby making it accessible for further analysis.

For an analysis, the velocity of the compression wave v_(p)(T), therelative energy E(T) of a measured signal, and the frequency spectrumf(T) of the signal can be extracted with corresponding algorithms. Thevelocity of the compression wave v_(p)(T), the relative energy E(T) of ameasured signal and the frequency spectrum f(T) of the signal depend onthe time T elapsed since production of the material and form together acomplete parameter set which contains the entire information about thematerial which can be obtained from elastic waves.

The wave velocity of the compression waves in the material can bedetermined from the quotient between running distance s and running timet(T) of the waves according to v_(p)(T)=s/(t(T)−t₀). While the runningdistance s, determined through the dimensions of the receptacle, isconstant, the running time t(T) of the signals is reduced withincreasing solidification of the material during the duration T of thetest. In this calculation, constant parts for the running time of thewaves through the walls of the container and for the time delay, causedby the measuring means, must be subtracted from the determined runningtime. This dead time t₀ of the system which is not related to thematerial can be determined through calibration measurement, which can beachieved in the most simple fashion through running time measurementwith direct coupling of transmitter and receiver container walls.

The relative energy E(T) is defined as a quotient of the wave energywhich can be measured after passage of the wave through the material,and the energy which was introduced into the material by the ultrasoundpulse. The individual energies are thereby calculated from the integralof the amplitude squares of the respective signals. If the introducedenergy cannot be used as measuring value, it can be assumed to beconstant when using a suitable ultrasound transmitter. The relativeenergy increases with increasing hardening or solidification of thematerial. The energy can further be represented as its integralovertime.

If the utilized ultrasound transmitter can generate sufficiently shortimpulses, the transmitted ultrasound wave contains more than one certainfrequency. A broad continuous frequency spectrum up to a certain limitfrequency is excited which is reciprocal to the impulse duration.Depending on the hardening or solidifying state, the material cantransmit different frequency portions in a different manner. After themeasurement, the spectrum of the signals can be calculated throughFourier transformation. If these individual spectra are normalized totheir maximum, added chronologically and the spectral amplitudes aregraphically represented as grey values, one obtains so-called contourplots. This three-dimensional representation permits calculation offrequency time curves or frequency time areas per individual measuremente.g. through calculation of average frequency maxima. Theserepresentations permit tracking of the spectral transition properties ofthe material as a function of time.

Correlation with previous measurements or with existing reference curvesfor velocity and energy produces e.g. findings concerning thecomposition of the material.

The measured curve shapes are examined more closely with respect to useof ultrasound technology within quality control with the aim ofmodelling the variation of the measured values (velocity, energy,frequency) with time in dependence on the material composition andnature. This is thus the solution of an inversion problem with unknownmaterial properties. The inventive method facilitates classification ofthe material within quality control after adjustment to the respectivetask.

To achieve this object, functions with sufficient free parameters mustbe used by means of which the curve shapes which are typical for thechange of the measured variable vp, E and f can be interpreted. TheBoltzmann function which is known from thermodynamics is e.g.particularly suited for the velocity:${y\quad (x)} = {\frac{A_{1} - A_{2}}{1 + e^{\frac{x - x_{0}}{dx}}} + A_{2}}$

It contains the four free parameters A₁, A₂, x₀ and dx whose values canbe used for adjusting the compensating function to the measuring curves.The quality of the inversion curves calculated e.g. for the velocity ismore than sufficient for the practical application of the method. Allfour free parameters can be used for a detailed classification of thematerials. The parameter A₂ e.g. can be associated with the water/cementvalue W/Z when examining unset concrete.

In a further development of the inventive method, an other embodiment ischaracterized in that the arrival time of an ultrasound wave (initialuse) is determined automatically with an algorithm which is based on thesum of the partial energy of the digitized received signal, wherein theenergy course S_(i) of the digitized signal is determined by the sum ofthe amplitude squares x_(k) ²:$S_{i} = {\sum\limits_{k = 0}^{i}\quad x_{k}^{2}}$

wherein x_(k) is the k^(th) sample point of the digitized signal and theminimum of the energy course S_(i) is determined which results fromcorrection of S_(i) with a trend δ:$S_{i}^{\prime} = {{\sum\limits_{k = 0}^{i}\quad x_{k}^{2}} - {i\quad \delta}}$

with ${\delta = \frac{S_{N}}{\alpha \cdot N}},$

wherein S_(N) is the partial energy at the last sample point N and α isiteratively determined through comparison of the corrected energy courseS_(i)′ with the measured wave shape of a received ultrasound signal, andthe arrival time of the ultrasound wave (initial use) is associated withthe minimum of the corrected energy course S_(i)′.

In this embodiment, the arrival time of the ultrasound signal at thereceiver is determined thereby producing the running time. To determinethis so-called initial use, an algorithm has been developed which isbased on the partial energy and the use of the Hinkley criterion, whichpermits a robust and very simple approach for initial use detection. Thesum of the partial energy S_(i), of an individual digitized wave signalcan be represented as sum of the amplitude squares x_(k) ² as below:$S_{i} = {\sum\limits_{k = 0}^{i}\quad x_{k}^{2}}$

The sample point i thereby corresponds to a certain time during thesignal. Arrival of the signal is thereby characterized by a significantrise of this energy sum. With respect to the algorithm, this means thatthe minimum of the sum curve must be automatically recognized frompartial energy minus a negative trend 8 suitably selected according tothe signal noise:$S_{i}^{\prime} = {{\sum\limits_{k = 0}^{i}\quad x_{k}^{2}} - {i\quad \delta}}$

The trend may be represented e.g. as follows;$\delta = \frac{S_{N}}{\alpha \cdot N}$

S_(N) is the energy at the last sample point N. An automatic iterationroutine was implemented for the variable a value for adjustment to thesignal quality.

The inventive method can be carried out in industrial practice forreliable and easy continuous monitoring of the state of a solidifyingand/or hardening material by means of an inventive receptacle and aninventive ultrasound transmitter. The receptacle preferably comprises aU-shaped part (24) from a highly-dampening material and two rigidcontainer walls (22,23), from a material which permits emitting planewaves, for mounting an ultrasound transmitter (3) and an ultrasoundreceiver (4), wherein the shaped part (24) and the container walls(22,23) delimit a receiving space for the material to be examined,characterized in that the U-shaped part (24) is pressed by means ofconnecting elements (25) engaging on the two container walls (22,23),between the two opposite container walls (22,23). The ultrasoundtransmitter preferably comprises means for generating the ultrasoundpulses through acceleration of a sphere (8) to exert an impulse, havinga large frequency content, onto the wall of a receptacle, characterizedin that the means for accelerating the sphere (8) are formed by acompressed gas acting directly onto the sphere (8) or through anelectric lifting magnet moved towards the sphere (8).

The shaped part of the receptacle acoustically decouples the containerwalls thereby providing at the same time secure sealing of the receivingspace to prevent leakage of the material from the receiving space. Theconstruction by means of the connecting elements facilitates assemblyand disassembly of the receptacle into its individual parts forcleaning.

The ultrasound transmitter comprises means for accelerating a spherewhich are formed by compressed gas or a movable lifting magnet. Thispermits reproducible ultrasound generation with simple means.

Although the invention is described with respect to the hardening ofconcrete, the inventive method or parts thereof is/are not limited tothe examination of concrete but also applicable for other materials,composite materials, plastic materials etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of the experimental arrangement forexamination of a material using ultra-sound;

FIG. 2 shows a longitudinal section through an ultrasound transmitterfor carrying out the method of FIG. 1;

FIG. 3 shows a longitudinal section through another ultrasoundtransmitter for carrying out the method of FIG. 1;

FIG. 4 shows a representation of a receptacle for carrying out themethod of FIG. 1;

FIG. 5 shows a representation of the measured behavior of an ultrasoundsignal while carrying out the method of FIG. 1, and of the sum of thepartial energy of the signal with 3 different values for the trend;

FIG. 6 shows a representation of the measured behavior of the changewith time of the propagation velocity of ultrasound waves while carryingout the method of FIG. 1 on mortar having different additionalsubstances;

FIG. 7 shows a representation of the measured behavior of the changewith time of ultrasound wave energy while carrying out the method ofFIG. 1 on mortar having different additional substances;

FIG. 8 shows a representation of the measured behavior of the changewith time of the energy integral of ultrasound waves while carrying outthe method of FIG. 1 on mortar having different additional substances;

FIG. 9 shows a contour plot illustrating the measured behavior of thetime changes of the frequency spectra while carrying out the methodaccording to FIG. 1 on concrete, and also a frequency-time curve derivedtherefrom;

FIG. 10 shows a parameter study for adjusting a compensation function tothe dependence of the change with time of the propagation velocity ofultrasound waves while carrying out the method of FIG. 1.

DESCRIPTION OF THE EMBODIMENT

The principle of the measurement is schematically shown in FIG. 1. Areceptacle 1 contains a material 2 to be examined. An ultrasoundtransmitter or impactor 3 transmits an ultrasound pulse via the wall ofthe receptacle 1 into the material 2 to be examined. At the same time ittriggers the A-D converter card A which starts the measurement. After acertain running time, the ultrasound waves arrive at the ultrasoundreceiver 4. The ultrasound receiver 4 converts the acceleration signalinto the voltage values which are then digitized and stored by the A/Dconverter unit B. A pre-amplifier C is provided before the A/D converterunit B. The A/D converter units A and B are connected to an evaluationand control unit D. The changes with time of propagation velocity,energy and frequency of the ultrasound waves give information about thematerial properties. The ultrasound sensor 5 provided for checking isonly required if an impactor is used as excitation.

The ultrasound transmitter 3 a of FIG. 2 consists of a non-magnetic pipe6 whose end 7 facing away from the receptacle holds a sphere 8 offerritic steel through permanent magnets. This pipe end 7 is providedwith an electrically actuated solenoid valve 9 which permits exertion ofa compressed gas impact onto the sphere 8. This compressed gas impactremoves the sphere 8 from the permanent magnet, the sphere isaccelerated by the spreading gas in the direction of the pipe end 10facing the receptacle and hits the housing wall of the receptacle,thereby spreading a short broad-band ultrasound pulse. Upon impingement,the sphere 8 loses only part of its energy and the remaining impulsereturns it into its initial position where it is again held by thepermanent magnet. The bores 11 prevent compression of the air columnbefore the sphere 8 and thereby deceleration of the accelerated sphere8. To determine the speed of the sphere 8, there is a light barrierdirectly in front of the opening of the steel pipe 6 via which thesphere 8 hits the housing wall. A safety means acting onto the solenoidvalve prevents inadvertent triggering of the compressed gas impact. Aviewing glass permits monitoring of the position of the sphere 8 in theinitial position. CO₂ is preferably used as compressed gas by connectinga gas bottle to the solenoid valve 9. The gas pressure can be controlledand changed by means of a pressure adjustment means. The impulse energycan be varied either in this fashion or through changing the valveopening time. Selection of an individual compressed gas impact, delayedcompressed gas impact or multiple compressed gas impact is possible bymeans of the control device via the solenoid valve. The trigger signalfor the compressed gas impact can be triggered either manually or viaTTL trigger signals. The delay time for the delayed compressed gasimpact or time between two compressed gas impacts can be adjusted tobetween 1 s and several minutes.

FIG. 3 shows an ultrasound transmitter 3 b comprising a lifting magnethaving a coil form 12 and a displaceable armature 13. A spherical cap 15is fixed to the armature tip 14. A voltage impulse from a control devicesupplies current to the coil form 12 such that the armature 13 isaccelerated from its rest position. Just before the armature 13 reachesits maximum deflection, the spherical cap 15 meets the sphere 16 held bya fastening means (union nut) which transfers the impact as ultrasoundpulse onto the receptacle. A restoring spring 18 returns the armature 13into its rest position where it remains on a dampening seat plate 19until the next voltage impulse. The sphere 16 can be replaced by meansof the removable mounting means 17 to vary the contact time duringimpact and thereby the impulse width (frequency width). A connectingpiece 20 is produced from electrically insulating material. Thespherical cap 15 is connected to the armature in an electricallyconducting fashion. A voltage applied between the sphere 16 and thearmature 13 is short-circuited for the duration of the impact contacttime. This produces a trigger impulse for external devices whose lengthcorresponds to the contact time. To obtain different impulse strengthsor energies, the length of the voltage impulse can be changed in thecontrol device. For this purpose, lifting magnets of different powerscan be used.

The receptacle 21 in accordance with FIG. 4 has 2 container walls 22, 23from a rigid transparent material between which a U-shaped part 24 ofelastic material (e.g. rubber) is disposed. The rigid container walls22, 23 are interconnected via connecting elements 25 thereby fixing theelastic shaped part between them. An ultrasound transmitter 3 isdisposed on the container wall 23 opposite to an ultrasound receiver 4mounted to the container wall 22. The receptacle 21 can accommodate ahardening and/or solidifying material to permit ultrasound examinationthereof in situ during hardening. The receptacle provides also contactbetween material and ultrasound transmitter 3 and receiver 4 via thecontainer walls 22, 23. The container 21 has hardly any effect on theexamination of the material since its acoustic properties are worse.Attenuation of the ultrasound waves in the container walls 22,23 and inthe shaped part 24 is larger than in the material to be examined. Thecontainer is constructed from few parts which are easy to handle andclean and additionally can be re-used. Due to their rigid form, thecontainer walls 22,23 produce radiation of almost plane waves, therebyomitting near-field effects. This permits on the one hand smallercontainer geometries (for point sources and propagation of sphericalwaves, measurements with a running distance smaller than double thewavelength would be problematic). On the other hand, the measurementaccuracy is increased since deviations in the centered arrangement ofultrasound transmitter 3 and receiver 4 have only a negligible influenceon the examinations. The shaped part 24 acoustically decouples thecontainer walls 22,23 and meets the task of sealing. The connectingelements 25 of which only one has a reference numeral, connect the freeends of the container walls in an acoustically non-coupling, elastic anddetachable fashion. The container walls 22,23 are pressed onto theshaped part 24. In addition, a rubber lid (not shown) can preventevaporation of water which would falsify the measurement.

FIG. 5 shows the principle of automatic detection of the time of initialuse for determining the compression wave velocity. The measured waveshape of an ultrasound signal is shown as an example. The sum of thepartial energy of the signal is shown with 3 different values for thetrend δ on the same time axis. One can derive therefrom that α=5 is mostsuited for determining the minimum energy, corresponding to the arrivaltimes of the waves. The initial use for α=15 is selected too early, forα=1 too late. The algorithm used produces this optimum result.

The following figures show exemplarily a representation of the change ofthe velocity of the compression wave v_(p)(T), the relative energy E(T)of a measured ultrasound signal, and the frequency spectrum f(T) of theultrasound signal as a function of time. FIG. 6 shows the change of thepropagation velocity of the sound waves with the example of mortarwithout and with three different additional substances. The change ofenergy is analogously plotted against time in FIG. 7. Both figures showthe rise of velocity or energy at different times corresponding to thedifferent properties of the additional substances. The quantity of therise and point in time when a certain final value of the velocity orenergy has been reached also varies. A variant is shown in FIG. 8, i.e.an integral of the energy wherein the slope of the curves show greatdifferences. The change of the frequency spectra is shown in FIG. 9 withrespect to measurement of concrete. The spectra show large low-frequencyportions at the start of the measurement and increasing broadening ofthe frequency band during further progress. The use of broad-bandultrasound sensors produces characteristic frequency-amplituderepresentations against time for different materials or materials withvarying elastic properties. From the frequency-amplitude representation,curves can be derived, which are easier to analyze. Determination, e.g.by calculation of the frequency maxima in the region of 0-20 kHzproduces the lower dotted curve of FIG. 9, which is typical for thismaterial. Such a curve can be determined also for further frequencyranges (e.g. 20-60 kHz) (upper curve). The area between the curvesdescribes characteristic material parameters.

FIG. 10 shows a parameter study for adjusting the Boltzmann functionselected as compensating function, to the behavior of the variation ofthe wave velocity in time. In this illustrated measurement with thedescribed method, the material was concrete. Corresponding to theirmathematical formulation${y\quad (x)} = {\frac{A_{1} - A_{2}}{1 + e^{\frac{x - x_{0}}{dx}}} + A_{2}}$

the four free parameters A₁, A₂, x₀ and dx are varied. The freeparameters are determined through optimum adjustment of the compensationfunction to the variation of the propagation velocity in time. Referencevalues of the free parameters are known from the reference measurementswhich correspond to certain material properties, such as rigidity,hardness, grain size or the like. When a material is to be examined, thecurrent values of the free parameters are determined and compared withthe reference values to obtain information about the properties of theexamined material.

What is claimed is:
 1. A method for examining a solidifying and/orhardening material such as cement, concrete, or the like, usingultrasound waves emitted by an ultrasound transmitter, wherein theultrasonic waves penetrate the solidifying and/or hardening material andare continuously measured and analyzed, the method comprising thefollowing method steps: i) recording the signal shapes of the ultrasoundwaves penetrating the material during solidification and/or hardening ofthe material; ii) extracting from the ultrasound wave shapes during theentire course of solidification and/or hardening of the material atleast one property selected from the group consisting of: a. the changewith time of the compression wave velocity, b. the relative energy ofthe ultrasound waves, and c. the frequency spectra of the ultrasoundwaves; iii) approximating the property of step ii) by a compensatingfunction having free parameters; iv) associating the free parameters ofthe compensation function with the properties of the material; and v)determining the material properties of the material by comparison of acurrent measurement with reference values of these parameters using thefree parameters of the compensation function.
 2. The method according toclaim 1, further comprising: determining automatically the arrival timeof an ultrasound wave (initial use) is with an algorithm which is basedon the sum of the partial energy of the digitized received signal,wherein the energy course S, of the digitized signal is determined bythe sum of the amplitude squares x_(k) ^(z):$S_{i} = {\sum\limits_{k = 0}^{i}\quad x_{k}^{2}}$

wherein x_(k) is the k^(th) sample point of the digitized signal and theminimum of the energy course S_(i) is determined which results fromcorrection of S_(i) with a trend δ:$S_{i}^{\prime} = {{\sum\limits_{k = 0}^{i}\quad x_{k}^{2}} - {i\quad \delta}}$

where ${\delta = \frac{S_{N}}{\alpha \cdot N}},$

wherein S_(N)′ is the partial energy at the last sample point N and a isiteratively determined through comparison of the corrected energy courseS; with the measured wave shape of a received ultrasound signal, and thearrival time of the ultrasound wave (initial use) is associated with theminimum of the corrected energy course S_(i).
 3. The method according toclaim 1 wherein the compensating function is the Boltzmann function. 4.The method according to claim 3, further comprising: determiningautomatically the arrival time of an ultrasound wave (initial use) iswith an algorithm which is based on the sum of the partial energy of thedigitized received signal, wherein the energy course S, of the digitizedsignal is determined by the sum of the amplitude squares x_(k) ^(z):$S_{i} = {\sum\limits_{k = 0}^{i}\quad x_{k}^{2}}$

wherein x_(k) is the k^(th) sample point of the digitized signal and theminimum of the energy course S_(i)′ is determined which results fromcorrection of S_(i) with a trend δ:$S_{i}^{\prime} = {{\sum\limits_{k = 0}^{i}\quad x_{k}^{2}} - {i\quad \delta}}$

where ${\delta = \frac{S_{N}}{\alpha \cdot N}},$

wherein S_(N) is the partial energy at the last sample point N and a isiteratively determined through comparison of the corrected energy courseS; with the measured wave shape of a received ultrasound signal, and thearrival time of the ultrasound wave (initial use) is associated with theminimum of the corrected energy course S_(i).